The purpose of these investigations is to examine the host factors involved in the pathogenesis is life-threatening bronchiolitis or pneumonia of infants due to respiratory syncytial virus or parainfluenza virus type 3. Both of these viruses produce the greatest morbidity and mortality in infants less than 6 months of age when virutally all possess varying levels of homotypic maternal antibody. Hypotheses have been advanced which implicate immune mechanisms in the pathogenesis of these infections, and these mechanisms directly or indirectly involve passively-acquired maternal antibody. It has also been suggested that sensitized lymphocytes may cross the placental barrier and enhance the severity of RS virus infections in early life. The proposed studies will examine the effect of passive immunization on the parameters of host response including circulating and surface antibodies, type 1 interferon, type 2 interferon and cell-mediated cytotoxicity which may or may not be antibody dependent. Previous studies of this model have demonstrated that passive immunization will suppress active antibody response and, probably, the production of type 1 interferon. Further studies are needed to determine the interrelationships of the other parameters. It is essential to define these relationships before proceeding with the development of any method of immuno prophylaxis in the infants less than 6 months of age who constitute the target population for such efforts.